Technical Field
The present invention relates to a flue gas treatment apparatus and to a flue gas treatment method for removing sulfur contained in combustion flue gas.
Background Art
Fossil fuels such as heavy oil, coal, and the like contain sulfur, and when such a fossil fuel is combusted in a boiler or the like, the sulfur contents are oxidized in a furnace and most of it is oxidized into sulfur dioxide (SO2) gas, and some of the sulfur dioxide gas is further oxidized to be converted into sulfur trioxide (SO3) gas. When SO3 gas is cooled in a smoke exhaust system, the cooled SO3 gas turns into sulfuric acid (SO3) mist. SO3 mist consists of condensed sulfuric acid, and if coal is used as the fuel, SO3 mist does not cause corrosion because SO3 mist is “sprinkled” over a large amount of coal ash; on the other hand, if a fuel containing low ash content such as oil is used, SO3 mist may often cause corrosion because the content of ash over which SO3 mist is to be “sprinkled” is low. In particular, if a fuel including very high sulfur contents such as heavy oil is used, a large amount of SO3 is generated, and thus, corrosion due to SO3 that may occur in devices such as a dust collection apparatus and a flue that are installed in the smoke exhaust system has become a serious problem.
SO3 mist consists of fine particles that are precipitated in a vapor phase and are difficult to collect even if a flue gas desulfurization apparatus is installed, and most of it may remain uncollected and pass through the gas desulfurization apparatus to be released from the stack as violet smoke. Therefore, a conventional flue gas treatment facility provided for power generation equipment in which the flue gas includes low ash content and the amount of generated SO3 is large (i.e., the conversion rate of S is high) has been known, in which calcium carbonate (CaCO3) is fed by injection for adsorption of SO3 at a location downstream of a denitrification apparatus, and thus, SO3 is separated and removed as gypsum to prevent corrosion of equipment that may occur due to SO3 (Patent Literature 1).
In addition, a conventional apparatus has been known, in which the temperature of the flue gas is lowered to a temperature at which SO3 in the flue gas changes into SO3 fumes, the SO3 fumes are adsorbed to charged solid particles such as calcium carbonate that has been electrically charged, and thereby, SO3 is separated and removed (Patent Literature 2). FIG. 3 is a schematic diagram which illustrates such a conventional apparatus.
In the apparatus illustrated in the drawing, calcium carbonate is injected by calcium carbonate feeding means 22 into combustion flue gas, the combustion flue gas is cooled by temperature lowering means 3, and thereby SO3 is condensed and adsorbed by calcium carbonate. In a desulfurization apparatus 5, the remaining calcium carbonate that has not reacted with SO3 is used as a material for the lime-gypsum method and SO2 is separated as gypsum.